1. Field of the Invention
The present invention is generally directed to a radar device and an on-vehicle radar device and in particular to a radar device and an on-vehicle radar device, in each of which demodulating of the reflected wave can be performed free from an output variation of the frequency-modulated transmission signal.
2. Prior Art
In recent years, environmental monitoring technologies such as vehicle cruising assisting devices, inter-vehicle distance control devices, collision alarm control devices, and automatic cruising control devices have been a focus of constant attention. In each of these devices, an extremely high frequency radar device is employed which uses beamed and reflected waves for measuring inter-vehicle distance (or a relative distance to an obstacle) and relative speed compared to a preceding vehicle. The calculated or measured distance and the relative speed are used in any one of an inter-vehicle distance control, a collision alarm control, and an automatic cruising control. As the extremely high frequency radar device, FM-CW radar devices (frequency modulated-continuous wave) have been widely used. In an FM-CW radar device, the transmission signal which is in the form of successive waves is frequency modulated at an FM-CW transmitter such that the waveform is triangle shaped. The resultant transmission signal is radiated, as a transmission wave, from a transmitting antenna.
The transmission wave radiated from the transmitting antenna is reflected by the obstacle and the resultant or reflected wave is received at a reception antenna. An FM-CW receiver mixes the reflected wave received at the reception antenna (i.e. reception signal) and the modulated transmission signal to produce a beat signal and extracts a relative distance frequency component and a relative speed frequency component to calculate a distance to the obstacle and a relative speed vs. the obstacle, respectively.
The aforementioned frequency modulation which shapes the wave into a triangle is performed at a frequency modulator. The frequency modulator is provided on a wave-guide which is made up of a dielectric-substance wire along which the successive waves are propagated from a Gunn oscillator to the transmitting antenna. The frequency modulator includes a varactor diode and adjusting a bias voltage thereof makes it possible to modulate the frequency in a substantially linear mode. In the aforementioned frequency modulation which shapes the wave there is an up-region in which the frequency is increased and a down-region in which the frequency is decreased. The frequency modulation is made to produce the up-region and the down-region alternately in repetition. That is, in the up-region the bias voltage is increased in gradual fashion to increase the frequency gradually. Then, in the subsequent down-region, the bias voltage is decreased in gradual fashion to decrease the frequency gradually.
However, in the FW-CW transmitter, the frequency-modulated transmission signal contains a low frequency component when the transmission signal is modulated at the frequency modulator, which generates an output variation in the transmission signal (i.e. power modulation). It is believed that this is due to inconstant output of the transmission signal relative to the bias voltage of the varactor diode.
Such an output variation of the transmission signal results in an imprecise beat signal produced by the mixture of the reflected wave (i.e. reception signal) and the modulated transmission signal at the mixer of the receiver. In brief, this means that it is difficult to establish high precision demodulation.
In view of such a circumstance, one solution would be to design the FM-CW radar device to employ a heterodyne system which is free from the transmission signal in demodulation. However, heterodyne systems are very complex in circuit design and are expensive to produce, resulting in that employing the heterodyne system becomes a costwise disadvantage.
Thus, a need exists to provide a radar device, for overcoming the aforementioned problems, in which a more precise demodulation can be made with an inexpensive circuit, and free from or independent of the output variation of the frequency-modulated transmission signal.
The present invention has been developed to satisfy the need noted above and a first aspect of the present invention provides a radar device wherein a transmission signal is subject to frequency-modulation, is radiated as a transmission signal from a transmitting antenna toward an obstacle, is reflected from the obstacle, and is received as a reflected wave or a reception signal, and on the basis of the transmission signal and the reception signal a beat signal is demodulated which contains a frequency component of a relative speed vs. the obstacle and a frequency component of a relative distance to the obstacle, wherein the radar device comprises a filter for deleting an output variation noise contained in the transmission signal which is frequency-modulated at the oscillator.
A second aspect of the present invention is to provide a radar device which comprises: an oscillator producing a frequency-modulated electromagnetic wave; a transmitting antenna radiating the electromagnetic wave as a transmission signal; a reception antenna receiving, as a reception signal, a reflected signal which is produced when the transmission signal is reflected from an obstacle; a directional coupler which extracts a local signal from the electromagnetic wave produced at the oscillator; a mixer mixing the local signal fed from the directional coupler and the reception signal fed from the reception antenna; an AC amplifier for AC-amplifying a mixing signal produced at the mixer; a demodulation circuit producing a beat signal which contains a frequency component of a relative speed vs. the obstacle and a frequency component of a relative distance to the obstacle, the demodulation circuit being in the form of a switching demodulation circuit; a switching circuit modulating the reception signal in switching mode before being fed to the mixer; and a high pass filter disposed between the mixer and the AC amplifier.
A third aspect of the present invention is to provide a radar device whose gist is to modify the structure as set forth in the second aspect, wherein the switching circuit is disposed between the directional coupler and the transmitting antenna.
A fourth aspect of the present invention is to provide a radar device whose gist is to modify the structure as set forth in the second aspect, wherein the switching circuit is disposed between the reception antenna and the mixer.
A fifth aspect of the present invention is to provide an on-vehicle radar device which comprises a radar device as defined in the first, second, third. and fourth embodiments wherein the radar device is mounted on an automotive vehicle and produces the beat signal for calculating a relative distance between the automotive vehicle and the obstacle and a relative speed as between the automotive vehicle and the obstacle.
In accordance with the first aspect of the present invention, the filter deletes the output variation noise contained in the reception signal which was originally contained in the transmission signal. Thus, a circuit which can be made inexpensive can demodulate a higher precision beat signal which is free from the output variation noise in the transmission signal.
In accordance with the second aspect of the present invention, the reception signal is, after being switching-modulated at the switching circuit, fed to the mixer. The mixer mixes this switching-modulated signal and the local signal from the directional coupler. At the high pass filter, the low frequency output variation noise of the resultant mixing signal is deleted. At this time, due to the switching modulation of the mixing signal, the low frequency beat signal fails to be deleted in switching modulation, and includes a frequency component corresponding to a relative distance to the obstacle and a frequency component corresponding to relative velocity. Thus, the beat signal component can be accurately demodulated.
In accordance with the third aspect of the present invention, the switching modulation of the reception signal is made at the switching circuit provided between the directional coupler and the transmitting antenna.
In accordance with the fourth aspect of the present invention, the switching modulation of the reception signal is made at the switching circuit provided between the mixer and the reception antenna.
In accordance with the fifth aspect of the present invention, mounting the radar device on the automotive vehicle makes it possible to establish at least any one of inter-vehicle distance control, collision alarm control, and automatic cruising control with much higher precision.